I'm not a paramedic. I'm not even a first aid instructor. I'm just a guy who's arrived first on a few scenes, and who listened when he was given first-responder first aid classes.
You won't live long if you don't breathe. While the rescue breathing portion of CPR is more and more being left out of the protocol, you're the new protocol still facilitates breathing, by clearing the airway and doing compressions. Breathing is essential, and it's still, for the time being, letter "A" in your A-B-C priority list. (Of course, if there's heavy bleeding, you're going to have to skip down that list a bit. Life is dynamic, you know?)
I bring this basic fact up, because time and again, I arrive on a scene to see something like the following: Man on the floor, on his back, unconscious, vomiting. Towels have been placed around him by someone else to attempt to clean up the vomit. The worried family member who has let me in has asked for no sirens because it will disturb the neighbors/the kids/Sumdood. As I lean over to assess the guy to advise our medics of his condition, a spew of vomitis, or saliva, or blood, or a combination of the three, is coughed up at me. Marvelous.
If you find a person who is unconscious, and cannot be revived, you need to protect that airway. Unless you know that it will cause other more serious harm (like with crushed ribs and/or spinal injuries), get them into the Recovery Position, right quick.
Simply put, the Recovery Position gets the patient on his side, with bent knee and outstretched arm to prevent rolling back over. The mouth should be slightly down cast to let matter drain. The chin is up to open the epiglottis.
And that's it. If you're of similar size, getting a person into the recovery position takes about four seconds. If they're much larger than you, then you're going to have to think about how to use their limbs as lever arms to get them onto their sides. If you ABSOLUTELY CANNOT do it, then call for emergency assistance right away, explain that you can't move them, and then go monitor the patient's airway, keeping it clear. If they're pregnant women, turn them onto their left side to keep the uterus off of the vena cava.
Look, some people have survived seizures, heart attacks, strokes, drug or alcohol-induced comas, etc--- all to die from the complications (like pneumonia) from aspirating fluids. Help them out, please.
Have a plan. When the emergency arises, put it into action. You can do this.